Recently, so-called portable video recorders in which a video camera and a VTR are miniaturized have been popularly developed; however, in the future, 8-mm video recorders in which a video camera and a VTR are integrally built will be highlighted as a further advanced form.
Such miniaturization of electronic equipment particularly largely depends upon the semiconductor technology. The photoelectric converting section of the above-mentioned video camera is also being replaced by solid-state image pickup devices in place of an image pickup tube owing to such advancement of the semiconductor technology. At present, these solid-state image pickup devices of this kind have many excellent characteristics as compared with conventional image pickup tubes.
In other words, as solid-state devices, they have many characteristics such as small dimensions, low electric power consumption, mass producibility, low heat generation and the like.
As described above, due to establishment of the technology of the solid-state image pickup devices having such many characteristics and due to development of supersmall-sized magnetic recording apparatus, the conventional silver salt photographic technology using silver salt films as a recording medium is being replaced by magnetic photographic or electronic photographic technology which requires no development processing.
If a main object of VTRs is to record a motion picture image in a VTR and to display it by a television, which is a principal method of use of present VTRs, is called a movie video, and if a main object is to record a still picture image in a recording apparatus and to display the recording signal on a television screen or to print it by a printer is called a still video, there is not a great difference in signalling format between the movie video and the still video since the signal is converted into a TV signal.
However, an object is in general continuously photographed in case of the movie video, while an object image is instantaneously photographed in the same manner as an ordinary camera in case of the still video. Therefore, it is necessary to make the operations largely different with respect to responsibilities of an iris, shutter, AGC, white balance, etc. and in addition, since their methods of driving the solid-state image pickup devices are different, there is a problem such that it is impossible to use a VTR provided with only the present optical and signal processing systems for both movie and still videos.
In case of commonly utilizing a camera section for both movie and still images, there occurs problems in particular with respect to methods of storage and readout of charges by solid-state image pickup devices. Although a MOS type device, interline type CCD (IL-CCD), frame transfer type CCD (FT-CCD), and the like have been known as solid-state image pickup devices, the FT-CCD will be described here as an example since their fundamental structures are similar and are not essential for the present invention.
As shown in FIG. 1, the FT-CCD comprises: an image pickup part 1 as a radiation receiving section consisting of a plurality of photoelectric converting picture elements for converting the radiation from an object image into charges; a memory part 2 for temporarily storing the signal charges from the image pickup part 1; a horizontal shift register part 3 for reading out the signal charges from the memory part 2 synchronously with the timing of a TV sync signal; and an on-chip amplifier part 4 for amplifying the signal charges from the horizontal shift register part 3 to output as a signal voltage. .phi..sub.PI and .phi..sub.PS denote vertical shift pulses in the image pickup part 1 and memory part 2, respectively, and .phi..sub.S represents a horizontal shift pulse in the horizontal shift register part 3.
In case of using such an FT-CCD as a movie camera, the photoelectric conversion is performed in the image pickup part in one field period as described above and these photoelectric converted signal charges are transferred to the memory part by the vertical shift pulses of several MHz in the vertical blanking period. The signal charges in the memory part are transferred to the horizontal shift register part in the horizontal blanking period at every one horizontal scan in the next field period, and they are read out as a CCD signal from the on-chip amplifier at the next stage. The image pickup part is in the photoelectric conversion state during this interval. Namely, the photoelectric conversion and vertical charge transfer are repetitively executed for every one field, thereby obtaining a continuous video signal.
In this case, since the first and second fields are interlacingly scanned by a television receiver, the information of both fields have to be formed in the mutually interlaced relationship at the time of the photoelectric conversion. Assuming that the image pickup part 1 has the vertical picture elements of the number which is twice the number of scanning lines of the TV receiver, such a problem as mentioned before will not be caused. However, it is extremely difficult to integrate the image sensors of such a number of picture elements in a limited space and to manufacture them.
Therefore as shown in U.S. Pat. No. 3,801,884, a method is considered whereby the position of the potential well is vertically shifted for every field by changing the level of the voltage to be applied to the vertical transfer electrode of the image pickup part 1 for every field.
According to this method, since the charge signal in the image pickup part is stored in the potential wells arranged under the different lines for every field, even if the number of vertical picture elements of the image pickup part is equal to only one field, it is possible to obtain the interlaced two-field signals, respectively.
However, with such a constitution, a problem will be caused such that the levels of dark current noises differ for every field.
This point will be described below. That is to say, the sensitivity of a camera is generally largely affected by a dark current relating to crystal defect of the image pickup device. FIG. 2 shows the relation between the dark current level of such a solid-state image pickup device and the drive pulse level. In the diagram, the ordinate denotes the output voltage of the dark current and the of abscissa indicates the drive pulse voltage. It will be appreciated from FIG. 2 that the dark current includes a bulk noise consisting of constant dark current components irrespective of the drive pulse voltage and a surface noise which increases with an increase of the drive pulse voltage.
In the case where the solid-state image pickup device is driven in the movie video mode conventionally, a drive pulse such as shown in FIG. 3 is supplied to the image pickup device to interlace a picture image as described above. In FIG. 3, T.sub.2 (T.sub.2 ') of .phi..sub.PI represents a charge accumulation period in the image pickup part and T.sub.2 (T.sub.2 ') of .phi..sub.PS indicates a charge transfer period from the memory part to the horizontal readout part. On the other hand, the information charges stored in the image pickup part are transferred to the memory part at a high speed in the period of T.sub.1 (T.sub.1 '). In FIG. 3, assuming that the charges accumulated in the period of T.sub.2 correspond to the signal of the fields bearing odd numbers and the charges to be accumulated in the period of T.sub.2 ' correspond to the signal of the fields bearing even numbers, the signal of the even-number fields has a drawback such that the dark current noise level becomes much larger than the signal level of the odd-number fields as is obvious from FIG. 2 since the drive pulse voltage is high at V.sub.H. Therefore, the S/N ratio of the signal would have changed for every field, causing a drawback of occurrence of flicker.